Method of initiating fractures in earth formations



SLAHUH www B. GILBERT Oct. 16, 1962 2 Sheets-Sheet 1 United StatesPatent O 3,058,521 METHUD F lNlTlATlNG FRACTURES IN EARTH FORMATlONSBruce Gilbert, Dallas, Tex., assignor to The Western Company of NorthAmerica, a corporation of Dela- Ware Filed Dec. 2, 1957, Ser. No.700,144 7 Claims. (Cl. 166--42) nainalnrrriaa-arid-minins.in.rherecogernorimdera.srgulnluidesastres..aetroleiirnmnatural..gas.-or...water, and A14 the1n `tu combustion of carbonaceous deposits SUC as 011, fari.rstralitemus shaleandsnatmeenam ground disposal of radioactive or otherwastes, and in other fields in which it is advantageous to createarticial fissures in the earths crust. For example, the method ofcreating fractures in earth formations according to the inventionfacilitates mining or quarrying operations by aiding in the separationof masses of rock or mineral from large formations. Similarly inpetroleum, gas or water recovery operations, the initiation andextension of artificial fractures in fluid-bearing earth formationsfacilitates drainage of such formations into a well from which thedesired fluids can be recovered. In petroleum production particularly,it is common practice after drilling a well bore hole into an oilIbearing earth formation to attempt to increase the productibility ofthe formation by creating therein artificial fractures which facilitatethe ow of petroleum from the formation into the well. For many years itwas the practice to try to increase the productibility of the formationby creating therein artiicial fractures which facilitate the flow ofpetroleum from the formation into the well. For many years it was thepractice to try to increase the productibility of oil-bearing formationsby horizontal drilling or by detonating massive charges of nitroglycerinin the well adjacent to the desired formation. Both of these practiceswere expensive, the latter being hazardous as well, and they have,therefore, been lsuperseded in recent years by generally more effectivehydraulic fracturing techniques such as those described in U.S. ReissueNo. 23,733.

In hydraulic fracturing methods, a liquid, of either hydrocarbon orwater base, is injected into the well under sufficient hydraulicpressure to cause the exposed formation at or near the bottom of thewell to break down, thus creating and extending fractures which increasethe productibility of the oil 'bearing strata. Sand or other granularmaterial is commonly incorporated in the fracturing fluid as a proppingagent to prevent closure of the newly created drainage channels by thepressure of the overburden of earth. When viscous liquids are used inhydraulic fracturing they often contain chemical additives which causethe viscosity of the liquid to decrease after a period of time so thatthe fracturing fluids can be readily recovered from the newly createdfractures by a uid produced from the formation. Hydraulic fracturingtreatments have come into widespread use since they are frequentlysuccessful in achieving a significant increase in the production of thetreated well.

Hydraulic fracturing treatments frequently produce only a singlefracture in an oil producing formation. When this occurs, it is usuallydue to the fact that availice able high pressure pumps are limited intheir capacity, making it impossible, as a general rule, to pumplsufficient fracturing fluid into the well to increase the treatmentpressure sufficiently to create a second fracture in the eX- posedformation while the initial fracture is taking uid. For this reasontechniques have been developed for isolating and treating short sectionsof a well lbore individually in order to produce fractures at differentlevels in the well when this is desirable. For example, a formation orcasing packer, either single or dual, can be run down the well casing ontubing and located and set in a Well to isolate and confine a selectedproducing zone which it is desired to fracture. This practice gives theoperator control over the general location at which an artificialfracture is created in an underground earth formation by means of ahydraulic fracturing operation. Multiple fractures may be created usingsufcient pump capacity. When this occurs, however, there is currently nomethod of controlling the direction and orientation of the fracture.Thus fractures may be extended out of oil bearing rock (or the desiredinterval to be fractured) into a barren, or undesirable zone; alsofractures may be extended into water bearing or gas bearing rockresulting in subsequent production of water or gas instead of oil. Alsomultiple fracturing frequently results in the formation of many smallfractures immediately adjacent to the well bore (or point of initiation)instead of a few fractures penetrating deeply into the reservoir. Alsofrequently a reservoir may be more efficiently produced by fractureslocated at strategic points. Current fracturing practices are random,and do not permit selection of where fractures are to be placed.

It has long been recognized that it would be highly desirable, not onlyto control the hydraulic fracturing operation so as to produce a singlefracture at the exact depth in the well at which oil has been determinedto be present but, in addition, to orient the plane of the fracture sothat it falls entirely within the producing formation. This has notheretofore been possible with the previously available hydraulicfracturing methods. Positive control of both the depth and theorientation at which an artificial fracture is created Would, of course,make it possible to create and extend only those fractures Which wouldbe most likely to provide the greatest increase in the useful productionof the Well, or which might otherwise produce the results desired, i.e.,formation of a horizontal cement barrier at a certain depth. The utilityof a fracture initiation method providing such positive control isapparent. For example, a more or less horizontal petroleum producingstratum is often located close to a similarly disposed water producingstratum. When a Well drilled through formations of this type isfractured by conventional methods, it frequently happens that therandomly created fracture is oriented in a more or'less vertical planecutting both petroleum and water producing strata and consequentlyflooding the well with Water. It is obvious that if it were possible tocreate a generally horizontal fracture at the desired depth in such aWell adjacent to the petroleum bearing stratum there would be nocommunication to the adjacent water bearing stratum and consequently thehydraulic fracturing treatment would produce a Valuable oil Well, ratherthan a worthless Water Well. The creation of fractures oriented inplanes disposed at various angles other than horizontal is, of course,frequently desirable particularly in mining and quarrying operations andalso in oil recovery operations where detection methods indicated thatan oil bearing stratum lies in such a plane. Also, in secondary andtertiary recovery operations it may be desirable to employ fractureslocated in planes other than horizontal. For example, gravity drainageproduction of reservoirs may be greatly facilitated by employing afracture at the bottom of a reservoir and extending slightly upward fromits intersection With the well bore. The method of the present inventionprovides for the creation of fractures in any predetermined orientationin an earth formation of any type.

Prior attempts have been made to control fractures in an oil well bore.One such early attempt consisted of firing simultaneously 4 shapedcharge shots spaced 90 apart in a single plane. The purpose was tocreate a horizontal fracture for squeeze cementing operations in theentire 360 around the casing in order to eliminate vertical migration offluid. Another attempt consisted of creating vertical fractures byperforating a formation through a well casing with shots arrangedlinearly 3 inches apart along one side of a gun. Another attemptconsisted of creating a wedge or circular cavity in the rock suroundinga Well bore in the hope that when hydraulic pressure was applied therock would break at the point of the Wedge. These efforts were allunsuccessful.

It is an object of the present invention, therefore, to provide a novelmethod for creating a fracture having a predetermined orientation in anearth formation such as concrete or the wall of a quarry, undergroundtunnel, or well bore hole.

It is a particular object of the invention to provide a method fortreating an earth `formation surrounding a Well bore hole to establishthe plane of any fracture initiated, created or extended therein by asubsequent hydraulic fracturing operation.

It is another particular object of the present invention to provide anovel method for initiating a fracture in an earth formation penetratedby a well bore hole at a single point of predetermined depth in theearth and in a predetermined orientation with respect to the bore holewithout the necessity of exposing a complete zone of the well byperforating.

It is a further particular object of the invention to provide a novelmethod for the creation of a generally horizontal fracture in an earthformation, i.e., a fracture in a plane more or less parallel with thehorizon, or in any plane, such as one about 45 from a plane parallelwith the horizon.

It is a further object of the invention to provide a method for reducingthe pressure required to fracture an earth formation by a hydraulicfracturing treatment.

It is a further object of the invention to provide a method for thepenetration of unusually thick cement sheaths surrounding a cased wellbore hole by creating a fracture therein, extending it into the adjacentformation.

It is a further object of the invention to provide a method forpreventing a hydraulic fracturing treatment from channeling between thewell casing and the surrounding cement sheath, and/or between the cementsheath and the surrounding formation.

Additional objects and advantages of the invention will be apparent fromthe following description of the method of the invention taken inconjunction with the accompanying drawing in which:

FIG. 1 is a diagrammatic sectional view of a cased well bore holeshowing cavities according to the invention in an oil bearing stratum;

FIG. 2 is a diagrammatic view of an earth formation containing aplurality of cavities arranged and spaced according to the invention;

FIG. 3 is a diagrammatic sectional view of the cavities of FIG. 2 takenalong the line 33.

FIG. 4 is a diagrammatic view of the earth formation of FIG. 2 after theapplication of hydraulic pressure to the cavities has established afracture therein;

FIG. 5 is a diagrammatic View of the fractured earth formation of FIG. 4taken along the line 5--5; and

FIG. 6 is a section along the line 6-6 of FIG. 1 after the fracture hasbeen initiated.

These and other objects of the invention are attained by creating in thesolid material, such as an earth formation, at the point to befractured, a plurality of critically spaced generally parallel holes orcavities oriented in the plane in which it is desired to create afracture and creating or extending a fracture in the plane defined bythe cavities by applying fluid pressure to them. The cavities are sospaced that the ratio of the diameter of a cavity to the width of theweb between adjacent cavities falls Within a critical range such thatwhen fluid pressure is applied in the cavities any fracture created inthe vicinity will be established in the plane defined by the pluralityof parallel cavities and extended in substantially the same plane.

More specifically, the invention is based on the discovery that when atleast two, and preferably three elongated, generally parallel, cavitiesare created in a single plane in an exposed earth formation and theratio of the diameter of an individual cavity to the thickness of theweb of earth formation between that cavity and the adjacent cavity orcavities falls within certain critical limits, it is possible toinitiate a fracture oriented in the plane defined by the parallelcavities by the application of sufficient hydraulic pressure to thecavities to fracture the formation. In the process of the presentinvention the plane of the fracture is controlled in a preoriented,predetermined plane by that of the cavity pattern and when the fractureis extended by hydraulic fracturing, the fracture is extended in thesame plane and not in some random unwanted plane which the user did notdesire. Any number of parallel cavities greater than one can be employedwith success although, as noted above, three cavities are preferredsince this number has been found to provide better and more consistentcontrol of the plane of orientation than two cavities. More than threecavities may be employed but it has been found that there is seldom anyadvantage in doing so since three provide satisfactory control offracture initiation in most cases. Also, in the usual small diameterwell bore holes it is difficult to introduce means for simultaneously'producing many more than two sets of three parallel cavities in a singleplane.

The diameter of the cavities, although not particularly critical, shouldbe great enough to afford a reasonably large internal surface in thecavity `for the application of fluid or hydraulic pressure and of courseshould not be so small that the uid or hydraulic pressure will beprevented from entering the cavity either by its own viscosity or by thebridging of solid particles in the fluid across the face of the cavity.There is no critical upper limit on cavity diameter as long as a web isleft between the cavities. It has been found, for practical purposes inthe majority of earth formations, that cavities having a ,diameter inthe range from about 0.25 inch up to several inches in diameter aresatisfactory. For Work in well bore holes which are generally from 4 to8 inches in diameter, it has been found that cavities ranging from about0.10 to 1.0 inch in diameter and preferably about 0.20 to 0.625 inch aresatisfactory.

The depth of the cavities is not particularly important so long as theyare relatively elongated with respect to their diameter. For example,cavities from 1 diameter to an infinite number of diameters deep, butdesirably 5 to 25 cavity diameters deep, and preferably 12 to 20diameters deep, are satisfactory in most instances. For use in Well boreholes, cavities which are about 0.30 inch in .diameter and 6.0 inchesdeep have been found to give excellent results.

The ratio of .cavity diameter to the thickness of the web of earthformation between adjacent cavities is a fundamental element of themethod of the present invention. It has been found that this ratio mayvary from about 0.25 up to, but not including, infinity, such as 1000 or100, or in other Words the cavities may be spaced from four diametersapart to almost touching. Of course if the plurality of cavities were indirect contact there would be no web between the cavities and the ratioof cavity diameter to web thickness would be infinity. For this reasonit is the maximum spacing between adjacent cavities which is of greatestimportance. The optimum ratio will depend somewhat upon the nature ofthe earth formation to be fractured. It has been found that when thecavities are spaced more than four diameters apart adequate control overthe orientation of a fracture initiated in the vicinity of the cavitiesis lost. The preferred ratio of cavity diameter to web thickness is inthe range from about 0.75 to 1.0. Ratios in this range are particularlysuitable for use in earth formations where the cavities must be producedin a well bore hole.

With a plurality of cavities placed in an earth formation in accordancewith the method of the invention the web thicknesses between thecavities are subjected to a concentration of stress which causes the webbetween the cavities to fail when subjected to hydraulic pressuresbefore failure occurs anywhere else in the exposed area. This orientedfailure provides initiation of a fracture in the desired predeterminedplane which is extended in the same plane upon the further applicationof fluid or hydraulic pressure. Because initiation of the fracture inaccordance with the method of the invention permits fracture extensionby hydraulic means at lower pressures than are necessary forconventional hydraulic fracture formation, it is possible to completethe fracture at sufciently W pressures that reduce the likelihood ofproducing new uncontrolled fractures in an unwanted or unoriented plane.This, of course, contributes materially to the success of the method ofthe invention. In the injection of the hydraulic breakdown fluid, it ispreferable to limit the pressure until a filter-cake can be developed bythe breakdown fluid preventing loss of fluid into and preferentialfracturing of intrinsic fractures or planes of weakness exposed to thebreakdown fluid. The fracture is then initiated between the cavities byincreasing the pressure until a break or a pressure drop occurs. Afterthis initial fracture is produced between the cavities, the pressurerequired to extend the single fracture in the desired plane is usuallyless than the pressure required to create the fracture.

The method of the invention may b'e employed in either cased or uncasedwells. Best results to date have been obtained in cased wells.

The cavities employed in the fracture initiation method of the presentIinvention can be produced in any manner since the method of theinvention is independent of the manner in which the cavities -arecreated. The production of the required cavities in Widely exposed earthformations such as the wall of a mine or quarry presents no problem andis most conveniently accomplished by drilling according to knownmethods, such as screw drills or pneumatic drills. When the workingspace is limited, as for example, in a deep well bore hole, the creationof cavities becomes more diicult. Such cavities can, however, beproduced lin a variety of ways including drilling, electric arcs,oxyacetylene torches, shooting with bullets, blasting with explosiveagents, the use of jets of corrosive or abrasive fluid, by means ofexplosive jet charges. Among the suitable cavity producing devices whichmay be employed are the rock piercing blowpipe described in U.S. PatentNo. 2,794,620 of C. S. Arnold et a'l. and the heat blasting fluid-jettool described in U.S. Patent No. 2,772,346 of T. I. Leston et al. Theuse of bullets and jet charges in the perforation of well casing, cementsheathing around such casing and the surrounding earth formations iscommon practice in treatment of oil and similar wells. Abrasive jetsbased on the sand blasting ,f principle would also be suitable as wouldjets of corrosive 6 jet charges is described in my copending applicationfor U.S. Letters Patent, Serial No. 675,424, filed July 3.1, 1957,entitled, Apparatus for Initiating Fractures in Earth Formations.

The cross section of the required cavities may take any form so long asit is generally rounded and is free of sharp angular crevices orconfigurations which might cause loss of control of the orientation ofthe fracture due to random failure of the formation in the angularcrevice. The preferred cross section is circular although yan ovoid orirregular cross section is suitable so long as it does not containangular crevices which would cause stress concentrations in a planeother than that desired.

Although the cavities are preferably generally parallel, it ispermissible for their `axes to converge somewhat and that the cavitieshave a tapered configuration so that the ratio is more or less constantalong their entire length.

The manner in which the method of the invention accomplishes orientedfractures in an earth formation will be illustrated in part by theattached drawings. FIG. l shows a cross section portion of anunderground earth formation 11 penetrated by a conventional well borehole containing well casing 12 anchored to the formation by lmeans ofcement sheath 13. The section of lthe well illustrated passes through anoil bearing stratum 14 of earth formation 11. Elongated cavities 15 in ahorizontal plane areshown penetrating the well casing 12, cement sheath13 and oil bearing stratum 14. It should be noted that although cavities15 are shown extending from opposite sides of `the well, the twocavities illustrated lie in the same Ihorizontal plane in which it isdesired to create a fracture. Although only one cavity 15 may be seen ineach side of the well in FIG. l it is essential that at least one othercavity and preferably two `additional cavities be associated with and inthe same plane as each of the cavities shown in the required spacing inorder to achieve the purposes of the invention.

FIG. 2 illustrates the open ends of three substantially parallelcavities 15 positioned in a single plane and spaced approximately onecavity diameter apart along a straight line in an earth formation 11.The open ends of the cavities are flush with the face of an exposedearth formation such as the wall of a mine or quarry or may beconsidered to be in the curved face of a well bore hole. The ratio ofthe diameter (a) of a cavity -15 to the thickness of the web (b) betweenadjacent cavities is 1.0 in the drawing since (a)=(b). In actualpractice this ratio can vary from 0.25 up to, `but not including,infinity, depending upon the characteristics of the formation.

FIG. 3 is a diagram-matic sectional view of the earth formation of FIG.2 taken along the line 3 -3 showing the depth of the cavities 15 inearth formation 11. The curved face of the well `bore hole is shown at16. The cavities 15 are shown to be about nine diameters deep which iswithin the preferred range of the present invention.

FIG. 4 is a diagrammatic view of the earth formation 11 of FIGS. 2 and 3showing an incipient fracture 17 filled with fluid 18. The vestiges ofthe faces of the cavities 15 may be seen as enlarged portions 19 of thefracture 17. The application of hydraulic pressure to the cavities 15 ofFIGS. 2 and 3 by means of hydraulic iiuid 18 has initiated the fracturein the formation in the plane established by the substantially parallelcavities 15 and established Ia fracture 17 which can be propagated inthe same plane by continued pumping of fracturing fluid.

FIG. 5 is a diagramma-tic view taken along the line 5--5 of FIG. 4showing the manner in which the fracture 17 in earth formation 11 wouldbe extended by continued pumping of fracturing fluid 18. Of course, thefracture 17 would be extended in the direction of the arrows as well asin all other directions in approximately a semicircle around the borehole 16 but for purposes of illustration the front of the fracture `isshown moving through the depth ofthe cavities 15 shown in broken lines.

liG. 6 is a section taken along the line 6 6 of FIG. 1 after thefracture has been initiated showing the fracture extending radiallyabout the full 360 around the well bore hole in a plane which issubstantially perpendicular to the -bore hole. The reference numeralshave the same significance as in FIGS. 1 and 5.

ln FIGS. 5 and 6 the cavities 15 are shown in broken lines to indicateonly the original position of the cavities rather than the cavitiesthemselves. As shown in FIGS. 5 and 6, the fracture has extended beyondthe length of the cavities.

The fracture initiated by the plurality of critically spaced cavities inthe single plane in the earth formation may be extended by introducinginto the formation under hydrostatic or iluid pressure any of thefracturing liuids which are known to those skilled in the art forIfracturing purposes. Preferred fracturing fiuids are the socalledlowpenetrating fracturing liuids. This type of fluid in general ischaracterized by its reduced tendency to penetrate the interstices of anearth formation. These iluids in general have a greater viscosity thanaverage crude oil or other well fluids or they tend to produce a plasteror cake on permeable formations with which they are brought intocontact. These fracturing fluids generally have a viscosity of from atleast 30 centipoises to about 5000 centipoises or even higher. Among thelow penetrating fracturing fluids which may be employed are thosedescribed in U.S. Reissue No. 23,733. Such uids may have either ahydrocarbon or water base. The hydrocarbon base uids are generally themost suitable for this purpose. These fluids are produced byincorporating a `bodying agent into a hydrocarbon fluid, such as crudeor refined oil. The bodying agent may comprise a colloid material or ametallic soap of an organic acid; a high molecular weight oleiinpolymer, particularly high molecular Weight linear polymers, such aspolypropylene; or an oil or water soluble plastering agent such as blownasphalt or pitch. Other bodying agents are well known to the art. Amongthe metallic soaps of organic acids which may be employed as the bodyingagent, are the hydroxy aluminum soaps. The bodying agent may be employedin suflicient quantity to impart to the base iiuid suicient viscosity orlow penetrability. The methods and materials employed to produce lowpenetrating fluids are Well known to those skilled in the oil welltreating industry.

As an example of the effectiveness of the method of the presentinvention on the operation of an oil well in the San Andres `formationin Texas, which on a production test prior to treatment in accordancewith the present invention produced two barrels of oil per day, the wellwas treated as follows:

The well, which consisted of a 4% diameter open hole having a depth of4,922', was first filled with 5,000 gallons of crude oil containingabout 1.2% by weight of sodium dodecylbenzene sulfonate containingcalcium sulfate as an inert filler. The well was next subjected tocavity formation employing a jet charge apparatus in accordance with myaforementioned copending U.S. patent application Serial No. 675,424.This apparatus comprised six shaped jet charge chambers in two opposingparallel rows of three chambers each capable of firing in oppositedirections, in which the jet charge chambers were 1 in diameter andabout 1% long lwith each charge in parallel alignment spaced 1.155 apartbetween the centers. The jet charge chambers were positioned in a planeparallel with the horizon. After simultaneous detonation of the six jetcharge chambers of the fracture initiating apparatus, the tiringmechanism was removed from the well, and the well permitted to standovernight. During this time, the iluid level in the well decreasedapproximately 330 feet below the surface. Approximately 8 barrels of thesame crude oil, containing about 1.2% by weight of sodium dodecylbenzenesulfonate which contained calcium sulfate `as an inert iiller, wereadded to bring the liquid level in the well to the surface. Fracturingiiuid in the amount of 1900 gallons of crude oil containing about 1.2%by weight of sodium dodecylbenzene sulfonate ('liuid loss additive)`containing calcium sulfate as an inert filler was next introduced intothe well. Of this 1900 gallons, the first 750 gallons contained no sandadditive. The remaining 1150 gallons contained 0.5 lb. of sand pergallon. In introducing this 1900 gallons of fracturing fluid, the first8 barrels were pumped before a surface pressure of 600 p.s.i. wasmeasured. The next 10 barrels were injected slowly at first with therate of pumping increasing to approximately 9 'barrels per minute at asurface pressure of 600 p.s.i. This injection rate at low pressureindicated that the fracture was initiated and at this stage theinjection rate Was increased to about 22.5 barrels per minute using thefluid containing the sand. After the 1900 gallons had been introducedinto the well, 4200 gallons of crude oil, containing sand as the onlyadditive, was introduced into the well. Of this 4200 gallons, the rst1500 gallons contained 0.5 lb. o-f sand per gal.; the next 1700y gallonscontained 1 lb. of sand per gal.; and the last 1000 gallons contained1.5 lbs. of sand per gal. During the addition of the oil containing sandas the only additive, an injection rate of 33.3 barrels per minute wasattained. Finally, 6300 gallons of ordinary crude oil was introducedinto the Well at a rate of about 29.5 barrels per minute to flush thefracturing and treating uids out of the well intothe formation.injection of liquid was continuous throughout `the fracturing operation.In this particular well, no initial pressure build-up and break wasobtained upon initiating the fracturing treatment, as is often the casewith the method of the invention. After approximately days a productiontest was conducted on the well and it lwas lfound that the well produced30 barrels of oil and 6 barrels of water per day. The results obtainedare particularly spectacular since wells in this region typicallyproduce water when employing other means of fracturing.

As will be apparent to those skilled in the art, the `orientation of theplane of -fracture may be positioned |by locating the plurality ofparallel cavities in a corresponding plane. This plane may be in anyposition with respect to the horizon from horizontal to vertical.

As has been expressed hereinabove, one of the important advantages ofthe process of the present invention is that it permits fracturing of anearth formation at a point where a fracture is desired at lowerpressures than were effective by prior hydraulic fracturing treatments.This 1s a direct contribution of the concentration of stressesintroduced in the web of the earth formation between the plurality ofsubstantially parallel cavities. Another concomitant advantage of themethod according to this invention over the prior hydraulic fracturingtreatments is that by initiating a plurality of parallel cavitles in asingle plane, a subsequent injection of a hydraulic iiuid is less likelyto channel Ibetween the casing 0f an oil well and its surrounding cementsheath and/or between the cement sheath and the surrounding formation.

The terms and expressions which have been employed are used as terms ofdescription and not of limitation, and 1t is not intended, in the use ofsuch terms and expressions, to exclude any equivalents of the featuresshown and described or portions thereof, but it is recognized thatvarious modifications are possible within the scope of the inventionclaimed.

What is claimed is:

l. A method of fracturing an earth formation in a predetermined planewhich comprises producing a plurality of spaced substantially parallelcavities in the plane in which the fracture is desired, said cavitiesbeing spaced sufiiciently close together so that the ratio of cavitydiameter to the thickness of the web of earth formation between adjacentcavities is at least about 0.25, and then injecting hydraulic fluid intothe cavities under such pressure as will initiate the fracture in theplane of the plurality of cavities.

2. A method of fracturing an earth formation in a predetermined planewhich comprises producing a pluralily of spaced substantially parallelcavities in the plane in which the fracture is desired, said cavitiesbeing spaced sufficiently close together so that the ratio of cavitydiameter to the thickness of the web of earth formation between adjacentcavities is from about 0.25 up to, but not including, infinity, and theninjecting hydraulic iluid into the cavities under such pressure as willinitiate the fracture in the plane of the plurality of cavities.

3. A method of fracturing an earth formation in a predetermined planewhich comprises producing a plurality of spaced substantially parallelcavities in the plane in which the fracture is desired, said cavitiesbeing spaced suliciently close together so that the ratio of cavitydiameter to the thickness of the web of earth formation between adjacentcavities is from about 0.75 to 1.0, and then injecting hydraulic fluidinto the cavities under such pressure as will initiate the fracture inthe plane of the plurality of cavities.

4. A method as defined in claim 1 wherein three cavities are employed.

5. A method as dened by claim 1 wherein the cavities are ofsubstantially circular cross-section of from about 0.25 to 1 inch indiameter and from about 1 to an infinite number of cavity diametersdeep.

6. A method as dened by claim 1 wherein the cavities 10 are ofsubstantially circular cross-section of from about 0.25 to 1 inch indiameter and from about 5 to 25 cavity diameters deep.

7. A method of fracturin,y an earth formation in a substantiallyhorizontal plane which comprises producing a plurality of spacedsubstantially parallel cavities in a substantially horizontal plane,said cavities being spaced sufficiently close together so that the ratioof cavity diameter to the thickness of the web of earth formationbetween adjacent cavities is at least about 0.25, and then injectinghydraulic iluid into the cavities under such pressure as will initiatethe fracture in the plane of the plurality of cavities.

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